Description: From Bacon and others (2014):"Cream-colored dacite pumice fall is exposed locally in cutbanks beyond the north flank of Aniakchak volcano. Soil from beneath this deposit 18 km north-northwest of the caldera at ~200 ft (60 m) elevation yields an age of 2,130+/-40 yr B.P., whereas organic sediment from beneath the deposit 14 km north of the caldera at ~300 ft (30 m) elevation gives an age of 2,300+/-80 yr B.P. (table 2 [in original text]). The weighted mean of these ages is 2,160+/-40 yr B.P. (table 1 [in original text]), but the large MSWD leads us to prefer the older of the two for a provisional age of ca. 2,300 yr B.P. The composition of a sample of pumice clasts (NA9413) does not exactly match that of any potentially correlative unit within the caldera but nevertheless is rather similar to that of West Dome. We [Bacon and others] tentatively consider the ca. 2,300 yr B.P. pumice fall to be the product of an explosive eruption within the caldera that preceded or possibly was contemporaneous with emplacement of the subaqueous domes that are described next."
"Within Aniakchak caldera, the oldest known products of postcaldera volcanism are four lava domes that were effused in a caldera lake. Relative ages of informally named Pumice, West, Vulcan, and Bolshoi Domes (fig. 10A [in original text]) are unknown. Pumice Dome is the most chemically evolved, two analyzed samples having 67.2 and 67.7 weight percent SiO2. Single samples from Vulcan and West Domes are compositionally similar at 65.6 and 65.8 weight percent SiO2, respectively, while a sample from Bolshoi Dome has 64.2 weight percent SiO2. The virtually identical compositions of the samples from Vulcan and West Domes suggest that they were emplaced at about the same time."
"Morphologies and surface features of the domes and the glassy character of the dacite lavas indicate that they were emplaced in water. The surface of Pumice Dome, poorly exposed other than in gullies and facing Surprise Lake, consists of pumiceous dacite in pillow-like forms ~0.3-2 m across (fig. 11A [in original text]) that are atypical of subaerial silicic lava but are consistent with extrusion into standing water. The surfaces consist of frothy, friable, light gray to white pumice characterized by cm-scale radial fracturing and irregular jointing (Allen and McPhie, 2000). Vugs as large as 20 cm across occur within pillows. Interstitial material is also pumiceous, banded, and radially fractured. West, Vulcan, and Bolshoi Domes are conical mounds of dacite vitrophyre whose flanks are coated by talus composed of finely vesicular to dense, polygonal joint blocks shed from outcrops on their upper portions. Outcrops on these three domes have fracture patterns (figs. 11B-F [in original text]) that are characteristic of chilling of degassed lava effused in water (see, for example, Goto and McPhie, 1998). Many structures and fracture patterns in lava flows and domes emplaced in contact with melt water within glaciers or adjacent to glacial ice (Lescinsky and Fink, 2000; Sporli and Rowland, 2006) closely resemble those in the domes at Aniakchak. Although hyaloclastite or breccia carapaces are not evident, 2-3-cm-thick patches of gray, sandy, and partially indurated tuffaceous sediment are found in pockets on the summit of Vulcan Dome. Thick glacial ice probably was not present at any time in the post-Aniakchak II caldera, whereas a deep intracaldera lake must have existed when the dacite domes were emplaced."
"Rhyodacite lava high on the northwest flank of Aniakchak volcano (fig. 4 [in original text]; labeled NW lava flow) is compositionally identical to a thin lava flow exposed within the caldera at ~2,500 ft (760 m) asl 1.5 km to the south, high in the west wall of the 1931 Main Crater (fig. 10 [in original text]; see also "Geochemistry" section [in original text]), above the level of any possible caldera lake. Little tephra is present and no Aniakchak I or II bombs have been reported on the blocky surface of the northwest lava flow where it has been examined, consistent with the flow being younger and similar in age to the lava in the caldera. The source of the intracaldera lava flow is unknown but appears to have been high on the caldera wall just inside the rim. The vent for the northwest flank flow must have been at ~2,400 ft (730 m) asl, 600 m west-northwest of the caldera rim. On the basis of their composition and apparent post-Aniakchak II age, these two flows are thought to have been approximately contemporary with Pumice Dome."
"Products of the earliest recognized postcaldera volcanism are small dacite domes extruded from the ring-fracture system into a deep caldera lake and a dacite pumice fall dated at ca. 2,300 14C yr B.P. The lake probably filled to its maximum of ~620 m above sea level (asl) within a few hundred years of caldera collapse. Pumice Dome (~67.5 weight percent SiO2) is compositionally similar to lava of the northwest flank flow and to lava exposed in the west wall of the 1931 Main Crater within the caldera, suggesting lateral transport of magma from a common source. Similarly, Vulcan and West Domes (~65.7 weight percent SiO2) are geochemically indistinguishable yet were emplaced on opposite sides of the caldera floor. Adjacent to Vulcan, Bolshoi Dome is the least evolved (64.2 weight percent SiO2). These postcaldera dacite-rhyodacites are not remnants of Aniakchak II magma but resulted from differentiation of recharge andesite or basaltic andesite magma."